1. Field of the Invention
This invention relates to castors and to vehicles which have castors. More particularly, the invention relates to a vehicle which has wheels by which the vehicle is propelled and steered, and castors which support a proportion of the vehicle's weight and which swivel to appropriate orientations in accordance with the direction of travel of the vehicle. Preferably the wheels are at the front of the vehicle and the castors at the rear.
It is to be appreciated that when we refer to a castor, we mean an assembly in which a ground-engaging wheel is supported not only for rotation but also for swivelling movement about an axis, herein called a swivel axis, which is substantially vertically oriented in use and which is offset from the rotational axis of the wheel.
The invention has been devised in relation to a vehicle which is intended to be occupied by one person, to give mobility to that person if he or she is disabled or infirm. Such a vehicle is commonly known as a wheelchair, and for convenience will herein be referred to thus.
2. Description of Prior Art
Wheelchairs are known in which the front wheels provide for driving of the wheelchair but do not take any part in the steering thereof, whilst rear castors have their swivelling controlled by a servomechanism in order to steer the wheelchair. The front wheels may be driven by a common motor and a transmission arrangement which includes a differential to permit the wheels to rotate at different speeds when the wheelchair is cornering. Such known wheelchairs have a disadvantage in that the angle to which the castors can be swivelled in order to steer the chair is limited; if the castors are swivelled to too great an angle the castor wheels just skid sideways and have little or no influence on the direction of travel of the wheelchair. Thus there is a limit on manoeuvrability of the wheelchair: to turn the chair to face in the opposite direction requires a "three (or more) point turn" to be executed.
It is known in wheelchairs generally to provide for steering by differential driving of the driving wheels of the wheelchair. For example, the driving wheels may have separate electric driving motors and the wheelchair may be provided with a control system which is arranged appropriately to control the supply of electrical power to the two motors for steering the wheelchair. If a wheelchair of this type had the driven wheels at the front and castor wheels at the back, the position of the centre of gravity of the wheelchair with its occupant, usually is such that when the vehicle is steered while travelling forwardly an "oversteer" condition tends to exist, in which the rear end of the vehicle, unconstrained by the castor wheels, swings outwardly on a corner relative to the front of the wheelchair. Further, the directional stability of the wheelchair is such that very careful operation of the controls is necessary for the vehicle to run straight ahead, as the rear castors do not contribute to the directional stability of the wheelchair. Nevertheless, steering by differential driving of front wheels which do not undergo any steering pivotal movement is advantageous, because driving one front wheel forwardly while the other either is not driven at all or possibly even is driven rearwardly enables the vehicle to be turned in little more than its own length.
Wheelchairs with wheels and castors are disclosed in U.S. Pat. No. 4,614,246 and GB-2,275,029. In the latter, the castors have their swivelling axes inclined for restraining their swivelling movement. A further wheelchair with wheels and castors is disclosed in U.S. Pat. No. 5,275,248. Castors with means for controlling or biasing their swivelling movement are disclosed in U.S. Pat. No. 3,924,292, EP-0,625,434 and DE-3,136,203, while a shock absorbing wheel suspension assembly is disclosed by U.S. Pat. No. 4,392,668.